Computer networks such as frame relay or X.25 networks transmit binary information from an originator to one or more recipients. An endpoint device such as a router receives the information from the originator and partitions the information into chunks of information known as frames. During the partitioning process, header information such as routing information is added to the frame based upon instructions from the originator. The router can partition the information into any size frames it desires, subject to a maximum allowable frame size for the network. The endpoint device then queues the frames and transmits them into the network one at a time.
The network uses the routing information to bring the information sent by the originator to a destination endpoint device that accepts information for a specified recipient. The destination endpoint device then reassembles the information from the frames and passes the information to the recipient. Each endpoint device is any device that can supply and/or receive information from the network, such as a conventional router with a frame relay interface or a conventional computer with a conventional frame relay network interface card installed along with the appropriate software.
Some endpoint devices, such as conventional routers, are shared by more than one originator. A router may receive information to transmit from a second originator before it has transmitted the information from a first originator. Because the frames from the two originators will share the same communication line from the router to the frame relay network, a conflict for the shared resource of the communication line can exist when more than one originator tries to use the endpoint device to send information. If the router can multitask, it can perform the routing and partitioning function while it transmits information from the first originator. However, because the communication line from the router to the network can only handle one frame at a time, the router will append the frames from the second originator into the queue for transmission at a later time, such as after the information from the first originator is transmitted into the network.
Some information is time-sensitive and therefore a simple first-in, first-out queue can cause the time sensitive information to be delayed longer than is acceptable. Such delay can produce undesirable results. For example, if the information sent to the router by the second originator described above is time sensitive, and the information sent to the router by the first originator is not time sensitive, the time sensitive information must wait until the non-time-sensitive information is fully transmitted.
Examples of time-sensitive information in a network are voice and video data. Voice data is time sensitive because the recipients may notice any delay in the conversation, and video data is time sensitive because, once started, any interruptions will be viewed as either "snow" or "jerky" motions. While some amount of delay is acceptable in both cases, the acceptable limit of the delay is finite and small. The acceptable delay can be exceeded due to a large data transmission behind which the time-sensitive information can be placed in the queue of the endpoint device. The acceptable delay is typically expressed in units of time, such as 50 milliseconds for voice data.
Some frame relay networks allow information to be prioritized in the queue. Time sensitive information received after non-time-sensitive information will nevertheless be framed, and those frames will be moved ahead in the queue of the endpoint device and transmitted into the network immediately, being delayed only for so long as to allow any partially transmitted frames to complete transmission.
In some circumstances, this delay to wait for partially transmitted frames to complete transmission can still be unacceptable if the time it takes to complete transmission of a partially transmitted frame exceeds the acceptable delay. The time it takes to complete transmission of any partially transmitted frame, referred to herein as transmission delay, is a function of the transmission speed of the endpoint device into the network and the number of remaining bits of the frame in the process of transmission when the time-sensitive information is received. In the worst case, the remaining number of bits is one less than the size of the frame.
One solution to this problem is to use an unshared endpoint device to transmit time-sensitive information. However, such a solution may be prohibitively expensive. Another potential solution is to obtain a very fast endpoint device and connection into the network for which the transmission delay for frames which are even the maximum allowable frame size would be below the acceptable delay. However, this solution may also be prohibitively expensive. Another potential solution is to immediately abort the transmission of the frame in process if time-sensitive information is received. However, such a solution would cause the recipient to request retransmission of the aborted frame, increasing the traffic on the network, potentially significantly. Another potential solution would be to reduce the frame size on the network so that, in all cases, the transmission delay will be less than the acceptable delay. However, the reduced frame size would mean an increased number of frames. Because each frame contains a certain amount of overhead in the header such as the routing information, the increased number of frames could significantly increase the network traffic.
It is desirable to allow originators of time-sensitive information to transmit frames within the acceptable delay using endpoint devices and network connections of any speed and that are shared with other information without significantly affecting traffic of a frame relay network.